Metal surfaces in cooling water systems and boilers are subject to high levels of corrosion due to various factors, including the high temperatures to which the metals are exposed. Heat exchanger surface (skin) temperatures can reach about 200° F., which accelerates the chemical reactions resulting in corrosion. In addition, the water used in such systems is often of moderately high alkalinity (about pH 8-9) and high hardness, having very high levels (e.g. about 200 ppm or more) of dissolved metal cations such as calcium and magnesium.
Conventionally, cooling water in such systems is treated with phosphate-based corrosion inhibitors, which react with dissolved calcium to form calcium phosphate, which forms a precipitate/barrier deposit on the metal surface. Care must be taken, however, not to overfeed the phosphate, as too thick a layer of calcium phosphate impairs heat transfer. Typically, heat transfer degrades over time, and the affected equipment must be disassembled and cleaned periodically.